Deposition of thin films by a variety of sputtering methods are well known, and taught by, for example, U.S. Pat. No. 4,361,472 (Morrison, Jr.) and U.S. Pat. No. 4,963,524 (Yamazaki). Gases present in the apparatus may include O.sub.2, N.sub.2, H.sub.2 O, and CO.sub.2, among others. So-called "in-situ," thin film sputtering techniques have been used to deposit superconducting materials such as NbN and Nb.sub.3 Ge utilizing heated substrates, as taught by U.S. Pat. No. 3,912,612 (Gavaler et al.) and U.S. Pat. No. 4,043,888 (Gavaler), respectively. Electrically conducting, highly transparent films of In.sub.2 O.sub.3 have also been sputtered, as taught by U.S. Pat. No. 4,400,254 (Freller et al.). There, water vapor with a pressure of between 3.times.10.sup.-3 mbar and 5.times.10.sup.-3 mbar was used as the sole reaction gas, in combination with an inert gas and use of temperature sensitive, water-cooled substrates. These films are used as semiconductors. It was speculated that the water vapor either furnishes O.sub.2 for the oxidation of atomized In particles during decomposition in the high frequency plasma, or liberates a reducing compound which prevents excessive oxidation of the target and of the film being produced.
Since the discovery of YBa.sub.2 Cu.sub.3 O.sub.7 (1:2:3 superconducting oxide), many techniques have been investigated for preparing thin films of the compound, for example, rf (radio frequency) sputtering on a room temperature Y.sub.2 BaCuO.sub.5 substrate material, as taught by U.S. Pat. No. 4,929,595 (Wu). In Gavaler et al. in Physica B, Vols. 165 & 166, pp. 1513-1514, Aug. 16-22, 1990, "Optimization of T.sub.c and J.sub.c in Sputtered YBCO Films," both parallel and perpendicular rf and dc magnetron sputtering of YBa.sub.2 Cu.sub.3 O.sub.7 films are taught.
In Gavaler et al., Physica B, experiments were conducted in a non-baked vacuum chamber, evacuated to a base pressure in the 10.sup.-6 torr range, using SrTiO.sub.3, LaA10.sub.3, MgO, and ZrO.sub.2 substrates. Initially, pure O.sub.2 or a mixture of O.sub.2 +Ar were used as the reactive sputtering gases. The main residual gas as a result of the use of a non-baked deposition chamber was H.sub.2 O. Values of T.sub.c (critical temperature for superconductivity) of up to 91K were obtained. Water was deliberately added to the sputtering gas in certain parallel configuration sputtering tests in the amount of approximately 13.times.10.sup.-3 mbar (10 mtorr), With substrate temperatures at 670.degree. C. In the parallel substrate-target configuration, film T.sub.c 's of greater than 90K were obtained, which were generally about 5K better than films made without the addition of water under the specific experimental conditions described. The same H.sub.2 O addition using the perpendicular substrate-target geometry, which under the specific conditions described produced T.sub.c values of greater than 90K without addition of water vapor, produced no further improvement in the T.sub.c 's of the sputtered films, and in fact had, at that time the perceived negative effect of producing Cu-O particles in the films.
There has been a long-felt need to provide YBa.sub.2 Cu.sub.3 O.sub.7 films with good and reproducible superconducting properties. There has also been a need to slow the deterioration of T.sub.c 's of the films which are made from targets which have had long usage. It is one of the main objects of this invention to provide a process to solve such problems.